Girls should discuss that thing with each other first. Not to mention MC is forced to put Mallory in friendzone to get that option. That also means he is not interested in her - friends forever.
Well actually, it doesn't mean that. You can want both, but choose one, and that doesn't automatically mean that you are not interested in the other. In this scenario, you are interested in both, but you have to choose one, and you pick Jaye. If you play the game right (for this ending) Jaye loves and trusts you enough, and loves Mallory enough, to offer the opportunity to bring her into your relationship. It's entirely possible that the girls have discussed it in advance, as they have been spending a lot of time together and have both admitted that the other has become only their second real friend. Most of that is off-screen, so anything could have happened during that time.
Personally, I think that, although it
could have been done differently, this ending works well. It's more realistic than just "I want all the girls, and oh look, I'm going to get all the girls."
I think the issue here is that people are looking for a throuple (I'd like that too, don't get me wrong) but that's just not on the cards. Jaye in particular is not interested in that. So you can have a relationship where you are with both Jaye and Mallory, but they will not be involved with each other. It's not a throuple, so we shouldn't be comparing it to one and saying it's therefore bad in comparison.